CITRIC ACID 21 CHROMIC ACID Cr03 = 100 Pure chromic acid forms dark ruby-red, silky, acicular crystals, but that used technically is in much smaller crystals or in red crystalline masses. The impurities present are mainly sulphuric acid and alkali salts, more rarely nitric acid and barium and lead salts. 1. Sulphuric Acid.—I gram in 20 c.c. of water should give a clear solution, which is not rendered turbid by addition of hydrochloric acid and barium chloride. 2. Alkali Salts.—o-i gram is heated to redness and the residue treated with water and filtered; if the filtrate is yellow, alkali salts are present. 3. Nitric Acid.—i gram is dissolved in water and the liquid treated with sulphurous acid solution until it becomes distinctly green and then with ammonia ; the liquid is boiled and filtered, and the filtrate (which should be colourless) tested for nitric acid in the usual way. 4. Barium or Lead Salts.—A few grams are treated with water and a few drops of dilute sulphuric acid; after standing, the clear liquid is decanted and any residue examined for lead or barium sulphate. 5. Determination of the Chromic Acid.—About o-i gram is boiled with hydrochloric acid and the liberated chlorine collected in potassium iodide solution. The iodine set free by the chlorine is then estimated by means of sodium thiosulphate solution and starch paste (i c.c. of N-thio- sulphate = 0-03339 gram Cr03). Even " pure " chromic acid generally contains small proportions of sulphuric acid or sulphates. In the commercial product as much as 30% of potassium sulphate lias been found (Kraucli). CITRIC ACID C0H807 + HaO = 210 Large, colourless, odourless, non-hygroscopic crystals of acid taste, soluble in water and alcohol and, to a less extent, in ether. They may contain tartaric, oxalic and sulphuric acids and salts of calcium, iron and heavy metals (especially lead and copper). 1. Tartaric Acid.—i gram of the acid is dissolved in 2 c.c. of water and the solution treated with potassium acetate and alcohol: no turbidity should be produced. Very small amounts may be detected by dissolving i gram of the acid in 10 c.c. of distilled water and gradually pouring part of the solution into 15-20 c.c. of lime water, which should remain clear. 2. Oxalic Acid.—i gram of the acid, dissolved in 10 c.c. of water, should not be rendered turbid by addition of calcium sulphate solution. 3. Sulphuric Acid and Sulphates.—-The aqueous solution (i: 10) should not be rendered turbid by addition of barium chloride and hydro- chloric acid. 4. Lime.—The aqueous solution (i: 10), neutralised with ammonia, should not be rendered turbid by addition of ammonium oxalate. 5. Ash.—10 grams of the acid are carefully burned in a crucible andand empyreumutk*. substances.